The present invention relates to a development apparatus for developing latent electrostatic images formed in electrophotographic copying apparatus employing a one-component type developer.
The development method employing a one-component type developer has an advantage in that it does not necessitate a toner concentration detecting apparatus for maintaining a mixing ratio of toner and carriers as in the case of the development method employing a two-component developer. Therefore, the former method is advantageously used in inexpensive copying machines or copying machines which require minimum maintenance. Development method used with one-component type developers can be classified into two types. One is the type in which a low resistivity developer is employed, and the other is the type in which a high resistivity developer is employed. These two types are different in the development process. In the first type employing a low resistivity developer (hereinafter referred to as a low resistivity toner), the toner itself is not charged and accordingly, by use of a conductive rotary cylinder having a magnet therein, an electrically conductive path is formed between the photoconductor and the rotary cylinder through the toner, so that an electric charge, which is opposite in polarity to that of the latent electrostatic image formed on the photoconductor, is induced on the toner, whereby the latent electrostatic image is developed. However, this process has a disadvantage in that a finite time is required to induce an electric charge on the toner and the time cannot be reduced below certain limits. Accordingly, it is not suitable for high speed development and electrostatic transfer of a toner image by this process is difficult.
In the type employing a high resistivity toner, there are many variations. FIG. 1 shows one representative method of such type. In the figure, reference numeral 1 represents a photoconductor on which a latent electrostatic image is formed. There is a small gap maintained between the photoconductor 1, and the development roller 2, which is rotated in the direction of the arrow. Above the developer roller 2, there is situated a hopper 4 in which a toner 3 is held. As the development roller 2 is rotated, a necessary amount of the toner 3 is replenished from the hopper 4 to the development roller 2. The toner 3 carried out of the hopper 4 is triboelectrically charged to a polarity opposite to that of a latent electrostatic image by a triboelectric charging blade 5 which is in pressure contact with the surface of the development roller 2. The thus triboelectrically charged toner is then transported to a development station A. After development, the toner remaining on the surface of the development roller 2 is caused to pass through the hopper 4 again, to which a fresh toner 3 is added. The movement of the toner 3 is repeated within the hopper 4. In this case, however, the toner particles of the toner 3 are triboelectrically charged in contact with each other, so that each toner particle acquires a different polarity. As a result the toner particles electrostatically aggregate. Furthermore, since the toner placed in the hopper 4 is not electrically charged at all, it cannot be sufficiently charged by the triboelectric charging blade 5 when the toner 3 is replenished from the hopper 4 to the development roller 2 and is then triboelectrically charged by the triboelectric charging blade 5. In other words, even if a high resistivity toner is triboelectrically charged, it cannot be charged speedily and sufficiently in the conventional process. In order to solve this problem, a number of triboelectric charging blades can be employed. However, when a plurality of blades are employed, the toner layer on the development roller 2 cannot be made thick enough with the result that the image density cannot be sufficiently raised. Furthermore, this method has the disadvantage that only the superficial portion of the toner layer on the development roller 2 is charged.